1. Field of the Invention
This invention relates to a system for determining the deterioration of an oxygen concentration sensor and more particularly to a system for determining the deterioration of an oxygen concentration sensor caused by clogging of exhaust gas inlet in the sensor.
2. Description of the Prior Art
A number of techniques have been proposed for accurately measuring the oxygen concentration of exhaust gas from an internal combustion engine. As specific examples there can be mentioned in Japanese Laid-Open Patent Publication Nos. 61-272438 (U.S. Pat. No. 4,842,711), 61-272439 (U.S. Pat. No. 4,767,520, 61-294358, 62-3143, 62-96754 and 1-206251 and Japanese Laid-Open Utility Model Publication No. 64-32442.
Sensors of this type generally have two bodies each composed of oxygen ion-conductive solid electrolyte material disposed opposite each other and each provided with a pair of electric terminals so as to constitute an oxygen-pumping element and a cell element for detecting oxygen concentration. More specifically., the space between the oxygen-pumping element and the cell element is sealed off to form a gas diffusion chamber (diffusion restriction region). The wall of the chamber is provided with a slit or a hole for the introduction of exhaust gas, while ambient air is introduced on the opposite side of the cell element. The electromotive force developed between the terminals of the cell element is detected and compared with a reference voltage. A voltage proportional to the difference between the two voltages is applied across the oxygen-pumping element terminals so as to cause pumping current to flow from the external terminal toward the gas diffusion chamber terminal or vice versa and thus pump in or pump out oxygen ions. The pumping current is thus feedback controlled in the direction for reducing the difference between the electromotive force of the cell and the reference voltage. The pumping current value is converted to a voltage value proportional to the oxygen concentration. As a result it becomes possible to detect the air/fuel ratio over a wide range extending from a rich to a lean mixture.
However, in actual use of this type of oxygen concentration sensor for determining the air/fuel ratio of an internal combustion engine, the exhaust gas inlet slit tends over time to become clogged or plugged by the adhesion of slight or fine metal oxide particles contained in the exhaust gas. When this happens, the detection value deviates further from the actual value as the degree of clogging increases and this decrease in the detection accuracy makes it impossible to control the air/fuel ratio of the engine properly.
The object of this invention is therefore to eliminate the aforesaid problem by providing a system for determining the deterioration of an oxygen concentration sensor which prevents detection error by enabling any clogging of its exhaust gas inlet slit occurring in the course of use to be easily detected and, as a result, when applied for example in a system for controlling the air/fuel ratio of an internal combustion engine enables the air/fuel ratio to be converged on the target value with high precision.